An experimental study of the trapped electron instability using a feedback diagnostic

The dissipative trapped electron instability has been studied experimentally and theoretically in a linear device. Special emphasis is given to the effects of changing the length of the magnetic trapping cell (30 cm<2L<75 cm) and to the development of feedback as a diagnostic technique. It is shown that the dominant axial wavenumber k 2 , is determined by the length of the trapping cell, and this in turn has an effect on the real frequency omega R approximately=50 kHz. The minimum mirror ratio for instability onset is found to be R c approximately=1.2. The dissipative nature of the instability is also demonstrated using feedback. The mode is definitely identified as the dissipative trapped electron instability through the dependence of the growth rate on electron collision frequency mirror ratio and through the radial and axial localization of the mode. The saturation amplitude of the density fluctuations is compared with the measured growth rate to show that the growth rate is proportional to the square of the saturation level.